1. Field of the Invention
The present invention relates to magnetostrictive load sensors for electromagnetically detecting a load through the magnetostrictive effect.
2. Description of the Background Art
Magnetostrictive load sensors have conventionally been developed as sensors for detecting a load. A magnetostrictive load sensor detects a load by converting a change in the magnetic properties of a member to which the load is applied into a change in voltage, and outputting the change in voltage (refer to, e.g., JP 2003-57128).
A magnetostrictive load sensor according to JP 2003-57128 includes a sheet metal case, a detecting rod of a magnetic material, and a coil that is wound around a bobbin.
The case includes an upper case having an approximate bell shape and a receiving case having an approximate disk shape. The upper case is provided with an opening through an upper end, and also provided with a case flange extending around a lower end. An outer periphery of the receiving case is provided with four cut and raised portions at equal distances.
The detecting rod has a bar-shaped portion that extends in the vertical direction, and a rod flange that is formed around a lower end. The bar-shaped portion and the rod flange are integral with each other. A central portion of the bobbin that is wound with the coil is provided with a through hole that extends vertically along the axial center of the coil.
A method of manufacturing this magnetostrictive load sensor includes inserting the detecting rod into the through hole in the bobbin, arranging the rod flange of the detecting rod at a predetermined position (approximate center) of the receiving case, and covering the receiving case with the upper case. The case flange of the upper case thus comes in contact with the outer periphery of the receiving case. The upper end of the bar-shaped portion of the detecting rod projects upwardly through the opening of the upper end of the upper case.
In this state, the four cut and raised portions of the receiving case are bent onto the case flange of the upper case and caulked. In this way, the receiving case and the upper case are secured to each other.
The lower end of the upper case is provided with a notch. A cylindrical cover is formed at this notch which extends a predetermined length in a side direction of the case.
An outlet of a lead wire pulled from the coil inside the case is extended outside the case through the inside of the cover, in order to prevent a reduction in magnetic resistance and external disturbances. The lead wire outlet is thus shielded with the cover, and prevented from breakage and damage.
A magnetic switch for a starter according to JP 2002-313205 also adopts such a structure in which a lead wire that is pulled from a coil inside a case extends outside the case. In the magnetic switch for a starter, a terminal that is connected to the lead wire of an exciting coil is extended outside a switch case through the inside of a molded cover, which is connected so as to extend in a predetermined direction from the switch case.
In the aforementioned conventional magnetostrictive load sensor, the detecting rod is magnetized by flow of a current through the coil. In this state, application of a load to the upper end of the detecting rod causes deformation of the detecting rod and a change in the magnetic properties.
The change in the magnetic properties of the detecting rod subsequently appears as a change in the voltage generated across the coil. This enables a detection of the load that is applied to the detecting rod based on the change in the voltage across the coil.
In such magnetostrictive load sensors, a shift in the positions of a detecting rod and a coil inside a case causes variations in the sensitivities of the magnetostrictive load sensors. This reduces accuracy of load detection by the magnetostrictive load sensors, and reduces the reliability. Therefore, the detecting rod and the coil have to be positioned accurately inside the case.
However, there is a possibility that the rod flange is shifted from the predetermined position on the receiving case when the cut and raised portions are bent and caulked with the rod flange being arranged on the predetermined position of the receiving case.
The operation of securing the receiving case and the upper case by caulking, with care so as not to cause the rod flange to shift from its position, is very difficult and requires much skill. This makes the mass-production of magnetostrictive load sensors difficult.
Further, since the cover body for allowing the lead wire of the coil to extend outside the case has a longitudinal shape, it is difficult to reduce the size of a magnetostrictive load sensor.